Electronic instruments answering the general definition mentioned hereinbefore are already known, particularly from JP Patent Application No. 11-126115 A, JP Patent Application No 2001-175610 or EP Patent Application No. 1 134 630 A1 (corresponding to U.S. Pat. No. 6,625,087 —Paratte).
JP Patent Application No. 11-126115 A, and JP Patent Application No 2001-175610 disclose similar solutions consisting in providing the portable electronic instrument with a set of input/output terminals placed for example laterally on the instrument of the case. Each input/output terminal includes a connecting element mounted so as to be mobile with respect to the case. Each connecting element is adapted to occupy a first position (namely a non-pushed-in rest position) in which the connecting element is mechanically and electrically uncoupled from the corresponding transmission line inside the case and thus from the electronic unit. Each connecting element can occupy a second position (namely a pushed-in position) in which an electrical connection is established between the connecting element and the corresponding transmission line consequently allowing communication with the electronic unit. A specific adaptor is used to bring the various connecting elements into their second pushed-in position. In addition to the aforementioned input/output terminals, other similar terminals are for recharging the accumulator housed in the electronic unit with electric energy.
EP Patent Application No. 1 134 630 discloses another similar solution in which at least one push-button is used for transmitting and/or receiving data from the electronic instrument. In this case, the connecting element of the input/output terminal is formed by the stem of the push-button. The advantage of this latter prior art solution lies in the use of one or several control members (conventionally of the push-button type) for electrically connecting the electronic instrument to an external element, for example a personal computer. Consequently, no specific contact element is necessary for establishing an electrical connection between the electronic instrument and the external unit, this electrical connection being established as soon as the control member or members configured to have the aforementioned dual function are brought into the pushed-in position.
One advantage common to the aforementioned three solutions lies in the fact that, when it is not being activated, the connecting element ensures the mechanical and electrical uncoupling of the input/output terminals of and the corresponding transmission lines inside the case of the instrument. This ensures a first level of electrical protection for the interface between the electronic circuit housed inside the instrument case and the outside world.
This first level of protection is not, however, sufficient to ensure optimum protection for the interface. Thus, for example, although the connecting elements ensure the mechanical and electrical uncoupling of the input/output terminals and the transmission lines, there nonetheless remains a relatively significant risk of the electric charges being able to be introduced on the transmission lines during activation of the connecting elements. This risk is even greater if the electrical potential of the connecting elements is left floating in the rest state (in the non-pushed-in position).
Moreover, directly interfacing an electronic unit on the transmission lines cannot be envisaged. Indeed, the electronic unit is typically interfaced with other units, generally by means of a bus, which is shared by such components. If the electronic unit were directly interfaced on the transmission lines of the various input/output terminals, there would then be a significant risk of the inadvertent or non-inadvertent activation of the connecting elements causing interference on the bus (for example during a data read or write operation executed in a memory by a processing unit). If a control member is used to fulfil the function of the input/output terminal (like the solution disclosed in EP Patent Application No. 1 134 630 A1), this problem then becomes critical since the control member can be activated by the user at any time, in particular while the instrument is being handled in an electrically conductive medium (for example in water).
In order to respond to the problem of electrostatic discharge, using protective electrical components to establish a path for the discharge of the accumulated electrical charges is known. These are well known protective components, called ESD (“electro-static discharge”) or TVS (“transient voltage suppressor”) components. These components have, however, a major drawback insofar as they have a very high stray capacitance (of the order of 1 nF). If these components are placed on the lines in direct connection with the electronic unit, this stray capacitance will thus be present on the lines and will not only generate high power consumption but will also affect the response time of said lines.